The operating environment of the combustor of a high performance gas turbine engine is characterized by a number of hostile features. The combustor is exposed to the highest temperatures in the entire engine with local gas temperatures approaching 3,500.degree. F. Rapid and wide ranging thermal excursions during heat up and cool down of the engine result in the cyclic exposure of combustor components to thermal shock and to high thermal stresses. At operating temperature, the combustor liner must support a steep thermal gradient across the liner from the hot inner surface to the cooler outer surface. Although the combustor does not experience a high mechanical load, the large thermal distortion of the components under operating conditions requires that the combustor exhibit elevated temperature load-carrying ability. In addition, the combustor is subjected to hot corrosive gases which chemically attack and mechanically erode the combustor wall.
Advanced gas turbine designs have pushed the state of the art in temperature capability of metallic components to what appears to be a point of diminishing returns. New and exotic metal alloys can withstand higher temperatures than ever before, but are extremely expensive and contain strategic elements which are remarkably scarce. The highest performance combustor liners are limited to a surface temperature of about 2,200.degree. F. A high flow rate of cooling air must be directed over the metal alloy combustor liner surface during the operation of the turbine to ensure that the combustor wall temperature does not exceed the limitations of the metal alloy.
Ceramic materials are attractive materials for high temperature applications due to their characteristic high thermal stability. However, the use of ceramic materials in structures such as combustor burner liners has been severely limited by factors including fabrication development problems, the lack of fracture toughness that characterizes ceramic materials, and the extreme sensitivity of ceramic materials to internal flaws, surface discontinuities, and contact stresses. Conventional ceramic materials are thus prone to catastrophic failure when subjected to the thermal and mechanical stresses which characterize the combustor environment. Ceramic debris from a failed ceramic combustor liner can have catastrophic effects on downstream structures, such as turbine vanes or blades.
What is needed in this art is a combustor liner which overcomes the problems discussed above.